The present invention relates to a switched mode power supply, particularly for a television receiver. More specifically the present invention relates to a switched mode power supply which is provided with a protective circuit for limiting the current supplied to the primary winding of the power supply transformer.
In communications devices, such as, for example, a television receiver, it is known to generate the operating voltages required for the individual stages from the mains voltage by means of a switched mode power supply (for example see the article by R. Dangschat et al, "Ein Sperrwandlar-Netzmodul Mit Netztrennung", FUNKSCHAU, 1975, No. 5, pages 40-43). A switched mode power supply provides galvanic separation of the receiver circuits from the mains which is advantageous for the connection of external devices and for protectice insulation measures. Since a switched mode power supply operates at a frequency of about 30 KHz, which is high compared to the mains frequency, the separating transformer of the power supply serving to provide the galvanic separation can be designed substantially smaller and lighter in weight than a mains transformer for 50 Hz or 60 Hz. By providing a plurality of secondary windings or winding taps and rectifiers connected thereto, voltages of different magnitude and polarity can be generated on the secondary side of the separating transformer. A regulating circuit conventionally provided on the primary side of the transformer stabilizes the amplitude of these secondary voltages.
In a switched mode power supply, the amplitude of the current on the primary side and the amplitudes of the voltages on the secondary side are determined essentially by the above mentioned regulating circuit. If an error occurs in this regulating circuit, e.g. due to the failure of a component, the switched mode power supply may reached an undefined state and may generate unduly high currents on the primary side while producing unduly high voltages on the secondary side. This may endanger other components of the switched mode power supply and particularly the connected loads.
It is known to limit the primary current by connecting a limiter circuit including a Zener diode to the control electrode of a thyristor which controls the primary side switching transistor. The Zener diode limits the bias across the the control electrode of the thyristor and thus the maximum value of the primary current which depends on this bias. This protective circuit thus acts on the regulating circuit on the primary side. The proper operation of the regulating circuit and of the protective circuit thus depends on the proper behavior of various components. If one component, e.g. a coupling capacitor between the thyristor and the switching transistor, is not operational, the regulating circuit, and thus the protective circuit are no longer effective. The switched mode power supply may then reach an undefined state and may generate unduly high operating voltages on the secondary side.